Basic Electronics for Scientists and Engineers

174 Oscillators
+V cc
R L1 R B1 R B2 R L2
V cc
C 1 C 2
+ 0.6 V
+ 0V
T 1 T 2
Figure 7.3 The transistor astable
oscillator during state 1.
Note how this oscillator has the characteristics of a relaxation oscillator: nonsinusoidal
output (in this case, a sawtooth), timing set by capacitor charging,
non-linear component operation (in this case, the SCR, which is inherently nonlinear,
is the relevant component), and analysis in the time domain (we determined
V out as a function of time in Eq. (7.3)).
7.2.2 Transistor astable oscillator
Our next relaxation oscillator, the transistor astable oscillator, produces a pulse train
output voltage. The two transistors in this circuit, shown in Fig. 7.3, will alternate
being saturated (V ce ≈ 0) or cutoff (V ce ≈ V cc ) with each being in the opposite
state of the other. Thus both transistors in this circuit are operating non-linearly.
The output voltage can be taken off the collector voltage of either transistor.
We will break the operation of this circuit into four steps involving two stable
states and two transitions. Relevant current flows and voltages are shown for the
four steps in Figs. 7.3, 7.4, 7.5, and7.6.
During state 1 of the circuit, transistor T 2 is fully on (i.e., saturated) and T 1 is
off (i.e., cutoff). Since T 1 is off, there is no current in resistor R L1 and thus no
voltage drop across it. The collector voltage of this transistor, V c1 , is thus equal to
the power supply voltage V cc . Also since T 1 is off, we know its base voltage V b1
must be less than the 0.6 V necessary to turn the transistor on. On the other hand,
T 2 is on, so its base voltage V b2 ≈ 0.6 V. The base current is supplied through
R B1 . Since we assume T 2 is saturated, its collector voltage V c2 ≈ 0. The collector
current comes from two sources: through R L2 and through R B2 and C 2 . This latter
path charges up capacitor C 2 .